I tweeted out my finding and the result was a flurry of information. The culprit here is butyric acid, actually an ester, which is indeed found in human vomit.
Butyric acid is a carboxylic acid found in rancid butter, parmesan cheese, and vomit, and has an unpleasant odor and acrid taste, with a sweetish aftertaste (similar to ether). Butyric acid is a fatty acid occurring in the form of esters in animal fats and plant oils. Interestingly, low-molecular-weight esters of butyric acid, such as methyl butyrate, have mostly pleasant aromas or tastes. As a consequence, they find use as food and perfume additives.(Interestingly, it's not such a problem in beers where Brettanomyces is also present, because that wild yeast can convert the esters to more pleasant tropical-fruit aromas.)
The curious question is where it comes from. One brewer reported that it can be produced by Lactobacillus in the presence of oxygen (lacto are a anaerobic bacteria). But as I dig around in my admittedly crude fashion, it seems more likely to come from Clostridium, another anaerobic genus of bacteria. It seems like a terrifically pernicious beast; there are some species of Clostridium tolerant of boiling temperatures. Even more unsettlingly, it likes carbon dioxide:
Carbon dioxide from both fermentation and artificial introduction has been shown to have a stimulatory effect on the growth of Clostridium butyricum (as well as other bacteria such as E. coli) . If sanitation issues allow for Clostridium to enter the brewery, CO2 purging may encourage butyric acid formation.It doesn't seem like lacto is a significant factor in all of this. So why do some kettle-soured beer develop the delightful flavor of vomit? My theory is that brewers are introducing Clostridium when they use grain to inoculate their wort during kettle-souring rather than a pure culture of lacto. Grain does indeed have lacto, but it has other stuff, too. (It's why sour mashes almost always produce gross ancillary flavors like garbage and, yes, vomit.) The pH drop eventually inhibits Clostridium, but perhaps not before it's had a chance to add its own charming compound.
This is all just noodling about--the reason, I'm told, blogs were invented--and I would love love love actual scientific info should anyone be in possession of such. One blogger's half-baked theories may make for an interesting three minutes' read, but it would be preferable to actually learn what's going on.
If you know, do tell.